Optical communication systems are a substantial and fast-growing constituent of communication networks. The expression "optical communication system," as used herein, relates to any system which uses optical signals to convey information across an optical waveguiding medium. Such optical systems include, but are not limited to, telecommunications systems, cable television systems, and local area networks (LANs). Optical systems are described in Gowar, Ed. Optical Communication Systems, (Prentice Hall, N.Y.) c. 1993, the disclosure of which is incorporated herein by reference. Currently, the majority of optical communication systems are configured to carry an optical channel of a single wavelength over one or more optical waveguides. To convey information from plural sources, time-division multiplexing is frequently employed (TDM). In time-division multiplexing, a particular time slot is assigned to each information source, the complete signal being constructed from the signal portions created for each time slot. While this is a useful technique for carrying plural information sources on a single channel, its capacity is limited by fiber dispersion and fiber nonlinearities.
While the need for communication services increases, the current capacity of existing waveguiding media is limited. Although capacity may be expanded, e.g., by laying more fiber optic cables, the cost of such expansion is prohibitive. Consequently, there exists a need for a cost-effective way to increase the capacity of existing optical waveguides.
Wavelength division multiplexing (WDM) has been explored as an approach for increasing the capacity of fiber optic networks. A WDM system employs plural optical signal channels, each channel being assigned a particular channel wavelength. Since each channel can itself carry plural information sources via time division multiplexing, additional optical channels increase capacity over a single channel system in proportion to the number of channels. For example, a four-channel WDM optical system has 400% greater capacity than a conventional single channel system. In a WDM system, signal channels are generated, multiplexed, and transmitted over a waveguide. At the receiving end, the WDM optical signal is demultiplexed such that each channel wavelength is individually routed to a designated receiver. Through the use of optical amplifiers, such as doped fiber amplifiers, plural optical channels are directly amplified simultaneously, facilitating the use of WDM systems in long-distance optical systems.
In order for a signal to be demultiplexed, a single optical channel must be selected from the multiplexed optical signal. Bragg gratings designed to reflect precise, narrow-bandwidth optical signals may be employed as the wavelength-selecting elements. To ensure that an optical signal is properly selected, the carrier wavelength launched by the laser transmitter must accurately match the reflection wavelength selected by the corresponding grating. This is particularly true in WDM systems employing many channels, often referred to as "dense" WDM, where channel spacings are on the order of a nanometer. Such WDM systems require precise correspondence of the optical signal wavelength to the wavelength selection element wavelength for each channel in order to avoid "crosstalk," i.e., interference between adjacent channels.
Previously, attempts have been made to stabilize laser transmitters against long-term drift in order to produce an optical signal source having a constant wavelength. The issue of aging-induced wavelength shifts in source lasers is discussed in Chung et al., Phot. Tech. Lett. Vol. 6, No. 7, July, 1994, pp. 792-795, the disclosure of which is incorporated by reference herein. In U.S. Pat. No. 5,077,816 to Glomb et al., a portion of the output of a narrowband laser source is transmitted through a weak fiber Bragg grating. The amount of the laser signal passing through the grating is detected, and the amount of electrical energy supplied to the laser is varied in proportion to the amount of light passing through the grating. In this manner, the deviation between the wavelength of the light and the wavelength of the grating is minimized. In U.S. Pat. No. 5,299,212 to Koch et al., the wavelength of a tunable laser source is controlled using a fiber grating as a wavelength reference. In this patent, a feedback control current is fed to the Bragg section of a multi-segment DBR laser such that the wavelength of the laser is a function of the feedback control current.
While these techniques may be useful for controlling the wavelength of the optical signal source, the patents are silent concerning the wavelength control of the wavelength selectors within the WDM optical system. Further, the patents do not stabilize the wavelengths of the gratings which are used as wavelength references for the source lasers. Since the wavelength band of maximum reflectance for Bragg gratings can change over time, failure to stabilize the grating wavelength can cause the optical source locked to the grating to undesirably drift with the grating, risking interference with adjacent channels. For example, in FIG. 5 of the Glomb patent, a first set of Bragg gratings is used as frequency selective taps while a second set of Bragg gratings is used to determine the frequencies of the laser transmitters. Such a technique is an "open-loop" strategy for wavelength control in WDM optical communication systems. In an open-loop system, individual devices may be calibrated to wavelength references or standards, but the devices are not dynamically standardized against each other, leaving open the possibility that the wavelengths emitted or selected by the various devices are not correlated to each other over the lifetime of the system.
Due to the potential discrepancy between a launched laser wavelength and a grating selector wavelength, the grating wavelength band of maximum reflectivity is designed to be much wider (more than an order of magnitude) than the modulated laser wavelength band. This greater width assures the "capture" of the appropriate laser signal wavelength. However, the width of a Bragg grating is not uniform throughout its reflection spectrum. As seen in FIG. 1, which plots log reflectance vs. wavelength for a typical grating, the width of the Bragg grating near the peak reflectance is significantly narrower than the width of the Bragg grating at low reflectance. Although the operational region of the grating is designed to be at the maximum reflectance, the approximately trapezoidal shape of the reflectance spectrum adversely impacts the interchannel spacing for a wavelength division multiplexed optical system. For example, aggregate reflection of all adjacent channels of as little as -20 dB relative to the selected channel can adversely affect the system performance. Consequently, the interchannel spacings must take into account the spectral width of the gratings at low reflectances, not just the maximum reflectance at which the gratings are designed to be used. Failure to adequately space the channels could result in channel crosstalk caused by overlapping adjacent gratings.
Although Bragg gratings can be fabricated to precisely and accurately reflect specific wavelengths, the reflection wavelengths can change with time. See, for example, Erdogan et al. J. Appl. Phys. Vol. 76 No. 1, July, 1994, pp. 73-80, the disclosure of which is incorporated by reference. To mitigate wavelength drift over time, accelerated aging of the gratings, described in Erdogan et al., may be employed. However, for dense WDM applications accelerated aging is a time-consuming process, particularly since the gratings must be carefully re-measured following aging. This additional step adds significant expense, and decreases manufacturing yield, of optical devices which incorporate Bragg gratings.
While gratings can be fabricated for reflection of precise channel wavelengths, such accuracy can result in a low grating manufacture yield rate. Because a wavelength division multiplexed optical communication system can use hundreds of Bragg gratings in various optical devices, it would be useful to employ a set of highly precise "master gratings" to which other, possibly less precise gratings, are correlated.
There is a need in the art for improved wavelength division multiplexed optical communication systems, particularly, systems which accurately correlate the wavelength selected by one or more optical selectors to a wavelength emitted by an optical transmission source. More specifically, when Bragg gratings are used as the optical selection elements, there is a need to ensure that the reflection wavelength band of the grating corresponds to the optical channel wavelength transmitted within the optical system. By dynamically matching the grating wavelength to the laser wavelength for the life of the communication system, the spectral width of the gratings can be sufficiently narrowed to permit small interchannel spacings, resulting in greater channel density within a given spectral region. Such stabilized Bragg gratings could be used to provide highly accurate optical wavelength selectors, add/drop multiplexers, and demultiplexers for WDM applications. Further, such techniques would create a closed-loop optical system, i.e., an optical system in which information about the incident optical channel is used to control the channel selector grating wavelength, or in which the optical channel is locked to the selector grating reflection wavelength, assuring accurate correspondence between optical channels and wavelength selectors.